The present invention relates to suction boxes in the forming section of a papermaking machine, or more specifically to a dual compartment vacufoil unit at the secondary headbox.
Generally, in the forming section of papermaking machines, a primary headbox discharges paper pulp on a fast-moving papermaking fabric and water is drawn out of the pulp through the fabric to set up the initial paper sheet. The paper sheet travels on the fabric through a series of table rolls or foils, which remove water through the hydrofoil effect, and then passes over suction boxes, which function to remove water from the paper sheet by utilizing a vacuum pump to obtain a pressure differential between the top and bottom surfaces of the forming paper sheet. The pressure differential draws water through the fabric into the suction box and holds the paper sheet tightly against the fabric and the fabric tightly against the suction box surface. Vacuum augmented foils, or vacufoils, may be used to remove water after the conventional hydrofoils and prior to the suction boxes in the papermaking process. Vacufoils remove water from the paper sheet through a combination of the hydrofoil effect and a pressure differential.
A secondary discharge process is required to produce linerboard grade or other multi-layer paper. To produce multi-layer papers, the secondary headbox discharges a secondary delivery of pulp onto the primary paper sheet as it is transported on the papermaking fabric. In various methods, the discharge occurs as the fabric travels over hydrofoils, single compartment vacufoils, suction boxes, or the free span between these dewatering elements.
The second discharge process is not without an important processing problem. Boundary air carried on the dry primary sheet is believed to cause a wind tunnel effect in the converging geometry of the secondary discharge. Boundary air is the air carried on the surface of the primary sheet which becomes trapped when the secondary discharge converges on the primary sheet. If the secondary discharge lands on the primary sheet as the papermaking fabric partly traverses a solid element, for example, a solid dewatering element box cover, and as the fabric partly traverses an open surface, for example, a slot in a dewatering element box cover, both water and boundary air are forced into the same space between the primary sheet and the secondary layer. As both water and boundary air are forced into the same space, a high speed frequency vibration results. The high speed frequency vibration is believed to cause surface defects, known as chattering, chopping, or barring on linerboard grade or other multi-layer paper in the cross-machine direction.
None of the methods discussed above have proven successful in eliminating the chattering, chopping, or barring problem. One method used to eliminate the problem has been to decrease the sheet consistency ahead of the secondary discharge by shutting off the vacuum pump, thereby allowing more water to remain in the sheet. While shutting off the vacuum pump cures the problem, it results in a wetter paper sheet at the rolls further down the processing line. A wetter paper sheet is more susceptible to damage during the rolling process, therefore production speed must be reduced to avoid damage to the paper sheet.
Deliberately wetting the primary sheet may reduce the chattering, chopping and barring problem, but wetting seals off the airspace through the primary sheet. Sealing off the airspace causes the boundary air to be evacuated from the sides of the paper sheet and not through the primary sheet, as desired for a smooth, consistent multi-layer paper sheet. Evacuating air from the sides of the paper sheet causes further defects on the paper surface. Additionally, a reduced fabric speed is required to process a wetter paper sheet, since a wetter sheet is more susceptible to damage during further processing. Accordingly, there is still a need for a device or method to eliminate these problems.
Accordingly, it is an object of the present invention to provide a vacufoil unit for use in forming linerboard grade or other multi-layer paper.
It is a further object of the present invention to eliminate secondary headbox chatter in the production of multi-layer paper without reducing processing speed.
It is a further object of the present invention to eliminate secondary headbox chatter in the production of multi-layer paper without wetting up the primary sheet.
Still another object of the present invention is to provide a method to produce multi-layer paper free from surface defects.